In private households and in industrial applications, there is a general need for information concerning the properties of fluid media. Fluid media may be any fluid consumables such as aqueous media in the form of aqueous solutions of chemical substances, or oils and the like. It is necessary to detect the properties of the respective fluid media in order to determine basic properties when using these media, to identify any changes occurring during operation, and specifically to detect any contamination by foreign matter in a given fluid medium. Depending on the detected measured values and the conclusions reached by analyzing said data, it is possible to decide whether the fluid medium still meets predetermined requirements in its current state, or whether measures for cleaning or for replacing the fluid medium are necessary.
A lucid example of a fluid medium and in particular of an aqueous solution is the detergent solution that is processed in a washing appliance (washing machine, washing device) for cleaning clothes or other objects. Prior art methods and apparatuses involving tests on a detergent solution are described below.
Document DE 217 557 A1 discloses a method and an associated apparatus for controlling the addition of detergents or rinsing agents in washing appliances, with various sensors being arranged in the washing appliance in order to determine the physical and chemical properties of the detergent solution. Any change in the rise of the measurement signal during the addition of detergent or rinsing agents is detected and analyzed by an electronic circuit for analyzing the output signals of the sensors. In combination with the results of analysis, it is possible to control the dosing of detergent for the washing machine.
Document DE 197 55 418 A1 discloses a sensor element and an apparatus for measuring complex impedances in materials, said sensor element having two electrodes which are made of a conductive material and arranged at a predetermined distance from each other. The two electrodes are covered with a thin insulating layer, said insulating layer being relatively thin in comparison with the predetermined spacing between the electrodes. The sensor element thus formed is largely insensitive to the ambient conditions whose properties are to be detected. The output signals from the sensors are subjected to further processing in an evaluation circuit, and the properties of a respective liquid between the electrodes can be analyzed during such processing. More specifically, it is possible for complex impedances to be determined and analyzed as a measure of the liquids' properties.
In the private and industrial spheres, clothes are generally cleaned fully automatically, to a large extent, using electronically controlled washing appliances that are also called fully automatic washing machines. One overall aim of a washing process is to obtain optimal results with minimal consumption of water, electrical energy, thermal energy and detergent. Present-day washing appliances use optical measurement methods for detecting the turbidity of a detergent solution, in order to determine a detergent concentration at least approximately, as well as other methods, but these generally require complex analysis of data. Turbidity sensors often have low levels of accuracy, and the measurement result can be easily distorted by deposits in the vicinity of the turbidity sensor. In washing appliances, such deposits frequently occur in the form of detergent and dirt residues.
Important parameters associated with washing programs are the water hardness, as a basic parameter, the detergent concentration and the washing activity. As regards the current hardness of the water fed into a given washing appliance (generally fresh water or drinking water), municipal water works usually state a value that is at least safe. Based on this information, an amount of detergent can be determined according to the dirtiness of the laundry. It is clear in this regard that only rough estimates can be made here, with which it is barely possible to dimension an optimal amount of detergent with real precision.
Precise determination of the “water hardness” is an important basis for efficient and economical cleaning of textiles, although the expression “water hardness” mainly relates to the concentration of alkaline earth metal ions dissolved in the water. “Total hardness” mainly refers to the calcium and magnesium ions dissolved in the water, and also to their anionic partners such as HCO3−. From a generalized perspective, detecting the properties of a fluid medium such as water involves all the substances dissolved in the water, as well as any substances that may affect the washing process.
The water hardness, and the dissolved substances it entails, are disadvantageous for washing processes and for processing that involves heating the water.
In municipal water works, the water hardness, in particular the “total hardness”, is determined under laboratory conditions by a quantitative analysis in the form of titration. In a sample solution containing a known substance in an unknown concentration, a known substance is added in a known concentration (a “standard solution”), and the volume of standard solution that reacts with the analyte is measured. Based on the consumed volume of standard solution, it is possible to calculate the unknown concentration of a particular substance in the sample solution.
It is clear that quantitative analysis by titration generally requires laboratory conditions that are complicated and costly due to the apparatus and equipment required, and it is also necessary to add a standard solution with a precisely determined concentration to the sample solution. Using quantitative analysis by titration is therefore reserved for a well-equipped laboratory and cannot be applied in this form for simple and quick detection of the properties of a fluid medium such as water, for example, in general industrial applications or in private households.
Since washing processes and the detergents used are increasingly seen in terms of environmental protection aspects, it is necessary to dose not only a particular amount of water but also a particular amount of detergent according to the dirtiness of laundry, and it is also necessary to take into consideration the properties of the water (generally drinking water). Systematic and correct dosing of detergent results in energy and water savings, as well as a lower pollutant load in the wastewater. The properties of the water are another criterion, besides the dirtiness of the laundry, because they have to be taken into consideration when dosing the detergent.
One aim in this connection should be that a device for detecting, for example, the properties of fluid media such as the water or detergent solution supplied to a washing appliance, is simple in design and reliable in operation. Ideally, every washing appliance should be equipped with at least such device so that the required amount of detergent can be determined precisely in the washing appliances. This requires a simple and cost-efficient design, preferably in the form of a module, as well as uncomplicated and therefore cost-efficient assembly, since it can be expected that when such detection devices for washing appliances are used, then large numbers of them will be involved.